Bis-carbonic acid monoaryl esters which are derived from aliphatic polyester-polyols can be prepared, for example, by reacting polyester-diols with diarylcarbonates in the presence of special catalysts according to German Patent Application P 2,651,639.2 which corresponds to U.S. patent application Ser. No. 845,174, filed Oct. 25, 1977. Products of this type are valuable starting materials for various block copolymers and are, therefore, readily transesterified, for example with bis-phenols. The compounds thus obtained can then be used for preparing, for example, polycarbonate/polyester block copolymers, as described in German Published Patent Specification No. 2,651,639.
It has now surprisingly been found that polyester-diols with molecular weights (Mn) of over about 250, preferably over 600, can be reacted with carbonic acid diaryl esters to give mixed aliphatic-aromatic carbonic acid esters at reaction temperatures up to 200.degree. C. in a manner such that bis-carbonic acid monoaryl esters of polyester-diols lengthened via --OCOO-- groups are formed.
This reaction proceeds smoothly and without side reactions such as, for example, etherification of the terminal OH groups (for this, compare Japanese Auslegeschrift (Japanese Published Specification) 6,911,377).
The molecular non-uniformity determined by the starting materials is insignificantly changed by the process according to the invention, i.e., the characteristic molecular weight distribution for a polyester-diol remains virtually unchanged, but is shifted towards higher molecular weights.